U.S. Pat. No. 5,234,652 to Woodhams et al, issued Aug. 10, 1993, describes processes for the melt phase extrusion of high molecular weight polyethylenes to produce extrudates having substantially both an increased modulus and increased strength in the flow direction. In the process described therein, the high molecular weight plastic material, at or near its melt temperature, is forced through a die having a converging passage so as to produce a highly oriented extrudate. The plastic material at the die interface is in substantially extensional i.e. plug flow through the converging die passage.
The invention described in U.S. Pat. No. 5,234,652 is based upon the proposition that polymer chains, when fully extended and oriented in parallel fashion, confer greatly increased strength and modulus to the resulting oriented extrudate. Although this concept has been extensively applied to fibers and films, attempts to apply this concept to thicker sections have been limited by the natural tendency of polymer chains to quickly recover their unstretched equilibrium conformations at elevated temperatures. This strain recovery often manifests itself in a phenomenon called die swell, in which the molten extrudate elastically retracts and expands as it exits the heated die. In U.S. Pat. No. 5,234,652, a process is described wherein a polymer is extruded in the semi-solid state, viz. melt phase extrusion under conditions that forcibly extrude and extend the flexible polymer chains in the flow direction and retain such imparted orientation in the extrudate. Under such melt state extrusion conditions at low extrusion temperatures, sufficiently high molecular weight and plug flow, the molecular relaxation times are sufficiently long that product orientation is largely retained during and after cooling to ambient temperatures. High molecular weight polyethylenes were identified as particularly suitable for this process.
Various extrusion processes are known for the continuous production of integral structural foam products. Of particular relevance is U.S. Pat. No. 3,764,642--P. E. Boutillier, issued Oct. 9, 1973. These processes use the so-called "Celuka die" and provide a high-density, rigid skin extruded product of desired size having an inner foamed core.
Whereas hydrostatic extrusion of polmers has been known for some time (N. Inoue, M. Nishihara, HYDROSTATIC EXTRUSION, Theory and Applications, Section 4, Polymers, Elsevier Applied Science Publishers, pp. 333-362, 1985) the process is normally restricted to ram extrusion which entails deformation of a billet under conditions similar to the hydrostatic extrusion of metals. The prior art with respect to extrusion dies is extensive, as will be understood from the published text by W. Michaeli (EXTRUSION DIES, Design and Engineering Computations, Hanser Publishers, 1984). However, the precise conditions for achieving steady smooth extrusion of highly oriented polymers without melt fracture or die swell are not generally known by those in the extrusion industry.
This invention reveals a method whereby most filled polymers may be readily extruded at or near their softening temperatures to directly produce a highly oriented profile.